If no one else will be hitting the table while you complete the
operation, and if you don't mind about missing a few sequence values
(i.e., having a gap), you should just need the following.
SELECT NEXT VALUE FOR sequencename FROM sometable;
That will tell you the next value the sequence wants to hand out.
DROP SEQUENCE sequencename;
Then reconnect with the property as given below, and
CREATE SEQUENCE sequencename START WITH n;
where n is the value you retrieved in the first step.
The reason this might cause gaps is that client connections will cache
sequence values, so the one you retrieve might not actually be the first
one that hasn't been used; it'll just be the first one cached by the
connection you're using. But if you do it this way, and nothing else is
connected in the meantime, then you won't get any duplicates.
As far as I can see, if you're the only connected client, this *should*
do it with no gaps: no other clients will have cached any sequence
values, so you'll retrieve the first one your connection has cached
(which will be the first one available), and then that's where your
sequence will start when you recreate the sequence. But I'm not
absolutely certain about that, and you might want to try some experiments.
If the sequence is being used for a primary key column (a sort of
auto_increment), then the other option is to
SELECT MAX(id) FROM sometable;
and then add one to this value to determine where the recreated sequence
should start. That will ensure no gaps.
James
On 22/09/15 19:47, Michael McAllister wrote:
Mujtaba
Thanks for this information. Seeing as I am using Phoenix 4.2, what is
the safe and approved sequence of steps to drop this table and
recreate it as you mention? Additionally, how do we ensure we don’t
lose sequence data?
Michael McAllister
Staff Data Warehouse Engineer | Decision Systems
[email protected] <mailto:[email protected]> | C:
512.423.7447 | skype: michael.mcallister.ha
<mailto:[email protected]> | webex: https://h.a/mikewebex
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On Sep 22, 2015, at 1:32 PM, Mujtaba Chohan <[email protected]
<mailto:[email protected]>> wrote:
Since Phoenix 4.5.x default has been changed for
phoenix.sequence.saltBuckets to not split sequence table. See this
<https://git-wip-us.apache.org/repos/asf?p=phoenix.git;a=blobdiff;f=phoenix-core/src/main/java/org/apache/phoenix/query/QueryServicesOptions.java;h=79776e7f688fc700275d0502e31646afe2bbcb1e;hp=4e8879b1b7a6358db2c1f9ccb4fa169394fec721;hb=18e52cc4ce2384bdc7a9c72d63901058e40f04ae;hpb=b82c5cbccdf4eb944238e69a514841be361bfb6d>
commit. For older versions you can drop sequence table and reconnect
with setting client side phoenix.sequence.saltBuckets property.
On Tue, Sep 22, 2015 at 11:14 AM, Michael McAllister
<[email protected] <mailto:[email protected]>> wrote:
Hi
By default SYSTEM.SEQUENCE is installed with 256 regions. In an
environment where you don’t have a large number of tables and
regions (yet), the end result of this seems to be that with hbase
balance_switch=true, you end up with a lot of region servers with
nothing but empty SYSTEM.SEQUENCE regions on them. That mans
inefficient use of our cluster.
Have there been any best practices developed as to how to deal
with this situation?
Michael McAllister
Staff Data Warehouse Engineer | Decision Systems
[email protected] <mailto:[email protected]> | C:
512.423.7447 <tel:512.423.7447> | skype: michael.mcallister.ha
<mailto:[email protected]> | webex: https://h.a/mikewebex
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